My Hydrangea has only 1 flower on it this year any ideas

Answers

When did you prune it, number one question, and how much?
If you pruned it at the right time and by the right amount, and it just hasn't flowered this year, it could just be the same problem a lot of people are having - too much heat and dryness earlier in the year, which prevented the plant from initiating the flowering process. But that does assume you pruned correctly.